Optical sensing is a common non-invasive manner to measure the various constituents which make-up food products. One often employed manner of optical sensing is performed using an infrared light source, fiber optic cable and photodetectors. Infrared light is passed through a food sample and light transmitted through the sample is measured and analyzed by photodetectors. The data generated by the photodetectors can provide an accurate measurement of a given sample's constituent make-up because each component has unique absorbance properties and thus a unique optical signature in the infrared range.
One application for this technology involves optical sensing in connection with the production and processing of dairy products. In the production of dairy products it is desirable to quantitatively measure various constituents of the product such as fat, water, solids and sugar content from a stream of flowing liquid. Measurement of these components involves passing the flowing liquid past light emitted from an infrared source and then measuring the light transmitted through the sample with photodetectors. The apparatus which provides a location to direct infrared light through a flowing sample stream is referred to as a flow cell. Light generated from an infrared source is transmitted through a fiber optic cable to a window area in the flow cell which is transparent to infrared light. Infrared light passes through the window area and then through the sample passing through the flow cell. Light that is not absorbed by the sample then passes through an opposite window area where it is received by a second fiber optic cable. The second cable transmits the incident light to a photodetector where the intensity of the transmitted light is measured. The various constituents of food products such as milk, salad dressing, cheese, and yogurt have unique absorbance spectra in the infrared range. Quantitative measurements of the constituents of the food products can be by carried out measuring the light transmitted through the sample at predetermined wavelengths in the near infrared range. Using a device as described herein, a food producer can continuously monitor the various levels of constituents in his product throughout the production phase.
In prior art systems, fiber optic cables transmitting infrared light are received in a flow cell by opposite cylindrical extensions positioned perpendicular to an axis parallel to the direction of flow of the sample. In the flow cells which are currently commercially available, the cylindrical extensions are made of polysulfone. Fiber optic cables are received in a tubular cavity which ends in a distal circular window area. In the prior art flow cells, the window areas are positioned at the end of extensions which radially extend from the sidewalls of the flow cell and into the conduit carrying the food product. The windows are positioned in this manner so that light only passes through a reduced sample section as compared to the diameter of the conduit. Such a reduced section is generally required because an adequate amount of infrared light cannot sufficiently penetrate a large distance through a sample so as to enable a photodetector to make accurate measurements. The optimal distance between the opposite windows in a flow cell is dependant on the product that will be measured.
Standard sanitary gaskets made of rubber are used at the interface between the conduits carrying liquid food products and the flow cells. These sanitary gaskets merit and receive close scrutiny because of the interest in keeping the food supply free from contamination. Any interface between component parts provides a location for the potential introduction of harmful contamination. Because of the concern with contamination, there is a voluntary approval program which certifies acceptable conduit system components such as gaskets for use in the dairy industry. Although flow cells used for optical measurements are not subjected to any specific government regulatory approval framework, the 3-A establishes voluntary guidelines governing the use of food conduits in the dairy industry. Many producers therefore require 3-A approval of all component parts which potentially could come into contact with a food supply including flow cells.
Standard operating practice in the dairy industry dictates that the conduits carrying food products and all the fittings used therein be thoroughly cleaned on a daily basis. Because flow cells are within the conduits and are in contact with the food products they must also be disassembled and cleaned after each use. Generally accepted design parameters for conduits designed to carry dairy products attempt to keep the interior surface of the conduits as smooth and even as possible. Any interruption of the interior surface, such as a crevice, provides a harbor for food products to accumulate, coagulate and spoil. The presence of coagulated milk or dairy products within a conduit provides a suitable environment for the growth of harmful bacteria which can contaminate an entire food supply passing through a particular conduit. Moreover, an accumulation can be abruptly released into the food product further contaminating the supply.
FIG. 1 depicts an exploded view in partial cross section of a flow cell which has been approved by the 3-A for dairy applications. The flow cell, generally designated by the reference numeral 9, is interposed between two conduits which carry a liquid food product and sealed in place using a pair of standard sanitary gaskets 10 and 11. Standard sanitary gaskets are also subject to 3-A approval. The gaskets are received in opposite circular recesses 12 and 13 located on opposite end walls of the tubular passageway. The flow cell depicted in FIG. 1 is a hollow cylinder having a diameter approximately equal to the diameter of the conduits 15 and 16 which respectively transfer the liquid food product to and from the sampling location. Located on sidewall 22 of the flow cell 9 are opposite circular openings. Opening 24 receives cylindrical member 26 and a second cylindrical member 28 is received in the same manner on the opposite side. Opposite cylindrical members 26 and 28 are made of polysulfone and transparent in the infrared spectrum. The members have central passages each which receives a fiber optic cable which in turn directs infrared light either to or from a window area. As illustrated in FIG. 1, member 26 has an interior passage 27 which ends at window area 29. When in use, infrared light passes through the window area and into the flowing sample. While a portion of the light is absorbed by the constituents in the sample, the remaining light is transmitted through the sample and falls upon the opposite window area 29a located on the end of cylindrical tube 28. The cylindrical members are positioned perpendicular to the direction of flow of the liquid food product to be measured as it passes through flow cell 9. As best shown in FIG. 2, at the base of each cylindrical member a seal is effected between surface 32 of the opening 24 and the exterior surface of the member. In the flow cell depicted in FIGS. 1 and 2 the seal employs an annular Teflon gasket 30 which is compressed into beveled circular surface 32. Exterior surface 34 of the Teflon gasket engages beveled surface 32 while the interior surface of the gasket simultaneously engages the exterior surface 55 of the cylindrical member to form a seal. Surrounding opening 24 and extending from the exterior sidewall of the flow cell is a hollow cylindrical extension element 38 which receives a spacer element 40. Referring back to FIG. 1, Teflon gasket 30 is compressed by rotation of a clamp 42 which has threads which engage opposite threads on extension element 38. As the components come together, surface 46 of spacer element 40 engages a washer element 48 which in turn engages Teflon gasket 30. At the same time, a flange 50 on cylinder 26 is also engaged by clamp 42 and is received in circular recess 58 on the opposite side of spacer 40. Compression of the gasket into the beveled surface forms a seal between the sidewall of the flow cell and sidewall 55 of cylindrical member 26. In addition to its function associated with forming the seal between the window and flow cell, spacer 40 aligns and retains the cylindrical tube in a position perpendicular to the direction of the sample flow. A first "O" ring gasket 52 is positioned between the clamp and the circular extension to prevent moisture from entering the cylindrical member. Fiber optic cable 61 is retained within the cylindrical member by a second clamp fastener 63 located adjacent to clamp 42. A second "O" ring gasket 65 is also provided to deter moisture from entering the assembly along the fiber optic cable.
It can be readily appreciated that the flow cell described herein and depicted in FIGS. 1 and 2 is a relatively complex assembly comprised of many parts. In addition to the problems and costs associated with the assembly and disassembly during the required cleaning procedures, the flow cell often exhibits a crevice in connection with the seal effected between the sidewalls of the flow cell and the cylindrical member. The presence of the crevice is related to the manner in which the components of the flow cell come together and can be exacerbated when the components are not precisely assembled. Even in instances where the components form true seal, there is often a narrow crevice or groove contiguous to the Teflon gasket at the interface between the cylindrical members and the interior wall of the flow cell. Despite efforts to minimize the incidence and size of such crevices, the occurrence of a small crevice is frequently manifested. The problems with the crevice are compounded because at its location adjacent to the cylindrical member, the crevice is further subjected to forces which result from the interruption of flow by the cylindrical member which extends into the flowing liquid. Existing crevices are thereby aggravated by the constant application of hydraulic forces. Crevices, or grooves are undesirable because food products can accumulate within them and a suitable environment is created for the growth of bacteria. It is evident from the foregoing description that prior art flow cells exhibit a number of disadvantages, particularly in dairy applications where it is necessary to frequently disassemble and clean all components in a conduit system.